The present invention relates to a releasable coupling for use in an injection device of the type that receives a syringe, extends it, discharges its contents and then retracts it automatically. Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Because of the stack-up of tolerances of the various components of the device, a certain margin of safety must be built into the activation of the release mechanism, to ensure that it is effective. The consequence of underestimating the safety margin is that the release mechanism may fail to operate even once the syringe contents have been discharged, which is unsatisfactory in a device that is supposed to retract automatically, particularly for self-administered drugs. On the other hand, overestimating the safety margin may mean that some of the syringe contents are discharged after the syringe has retracted, which results firstly in a short dose and secondly in what may be termed a “wet” injection. Wet injections are undesirable for the squeamish, particularly in connection with self-administered drugs.
UK patent publication nos. 2388033, 2396298 and 2397767 describe a series of injection devices designed to deal with this problem. Each makes use of a neat trick that delays the release of the syringe for a certain period of time after the release mechanism has been activated, in an attempt to ensure that the syringe has been completely discharged. The devices illustrated in UK patent application no. 0325596 make use of a two-part drive incorporating a fluid-damped delay mechanism that is particularly effective in ensuring complete discharge of the syringe contents. In each case, the device relies upon two unlatching mechanisms. The first unlatching mechanism initiates the fluid damping mechanism and the second releases the syringe from the actuator, allowing it to be withdrawn. The unlatching mechanisms are activated by components of the injection device having been advanced to nominal unlatching positions relative to the device casework. Unlatching mechanisms, including the mechanisms described in the present application, that are activated by components of the injection device having been advanced to nominal unlatching positions relative to the syringe are described in our concurrently filed UK application with publication no. 2414399.
FIG. 1 shows just such an injection device 110 in which a housing 112 contains a hypodermic syringe 114. The syringe 114 is of conventional type, including a syringe body 116 terminating at one end in a hypodermic needle 118 and at the other in a flange 120. The conventional plunger that would normally be used to discharge the contents of the syringe 114 manually have been removed and replaced with a drive element 134 as will be described below, terminating in a bung 122. This drive element 134 constrains a drug 124 to be administered within the syringe body 116. Whilst the syringe illustrated is of hypodermic type, this need not necessarily be so. Transcutaneous or ballistic dermal and subcutaneous syringes may also be used with the injection device of the present invention. Generally, the syringe must include a discharge nozzle, which in a hypodermic syringe is the needle 118.
As illustrated, the housing includes a return spring 126 that biases the syringe 114 from an extended position in which the needle 118 extends from an aperture 128 in the housing 112 to a retracted position in which the discharge nozzle 118 is contained within the housing 112. The return spring 126 acts on the syringe 114 via a sleeve 127.
At the other end of the housing is a compression drive spring 130. Drive from the drive spring 130 is transmitted via a multi-component drive to the syringe 114 to advance it from its retracted position to its extended position and discharge its contents through the needle 118. The drive accomplishes this task by acting directly on the drug 124 and the syringe 114. Hydrostatic forces acting through the drug and, to a lesser extent, static friction between the bung 122 and the syringe body 116 initially ensures that they advance together, until the return spring 126 bottoms out or the syringe body 116 meets some other obstruction that retards its motion.
The multi-component drive between the drive spring 130 and the syringe 114 consists of three principal components. A drive sleeve 131 takes drive from the drive spring 130 and transmits it to flexible latch arms 133 on a first drive element 132. This in turn transmits drive via flexible latch arms 135 to a second drive element, the drive element 134 already mentioned.
The first drive element 132 includes a hollow stem 140, the inner cavity of which forms a collection chamber 142 in communication with a vent 144 that extends from the collection chamber through the end of the stem 140. The second drive element 134 includes a blind bore 146 that is open at one end to receive the stem 140 and closed at the other. As can be seen, the bore 146 and the stem 140 define a fluid reservoir 148, within which a damping fluid is contained.
A trigger (not shown) is provided at the middle of the housing 112 and, when operated, serves to decouple the drive sleeve 131 from the housing 112, allowing it to move relative to the housing 112 under the influence of the drive spring 130. The operation of the device is then as follows.
Initially, the drive spring 130 moves the drive sleeve 131, the drive sleeve 131 moves the first drive element 132 and the first drive element 132 moves the second drive element 134, in each case by acting through the flexible latch arms 133, 135. The second drive element 134 moves and, by virtue of static friction and hydrostatic forces acting through the drug 124 to be administered, moves the syringe body 116 against the action of the return spring 126. The return spring 126 compresses and the hypodermic needle 118 emerges from the exit aperture 128 of the housing 112. This continues until the return spring 126 bottoms out or the syringe body 116 meets some other obstruction that retards its motion. Because the static friction between the second drive element 134 and the syringe body 116 and the hydrostatic forces acting through the drug 124 to be administered are not sufficient to resist the full drive force developed by the drive spring 130, at this point the second drive element 134 begins to move within the syringe body 116 and the drug 124 begins to be discharged. Dynamic friction between the second drive element 134 and the syringe body 116 and hydrostatic forces acting through the drug 124 to be administered are, however, sufficient to retain the return spring 126 in its compressed state, so the hypodermic needle 118 remains extended.
Before the second drive element 134 reaches the end of its travel within the syringe body 116, so before the contents of the syringe have fully discharged, the flexible latch arms 135 linking the first and second drive elements 132, 134 reach a constriction 137. The constriction 137 is formed by a component 162 that is attached to the syringe flange 120, so it will be understood that when the syringe 114 advances from its retracted position to its extended position, the component 162 advances with it. The constriction 137 moves the flexible latch arms 135 inwards from the position shown to a position at which they no longer couple the first drive element 136 to the second drive element 134, aided by the bevelled surfaces on the constriction 137. Once this happens, the first drive element 136 acts no longer on the second drive element 134, allowing the first drive element 132 to move relative to the second drive element 134.
One drawback associated with this arrangement is that the latch arms 135 are flexed by a constriction 137 through which the drive elements must pass, and which can therefore, at best, flex the latch arms 135 so that their outer extremities coincide with the outer surface of the second drive element 134. As the first and second drive elements 132, to move relative to each other, the latch arms 135 must flex further, so that their outer extremities coincide with the inner surface of the second drive element 134. This requirement introduces manufacturing difficulties and may also affect the reliability of the unlatching mechanism itself.
Because the damping fluid is contained within a reservoir 148 defined between the end of the first drive element 132 and the blind bore 146 in the second drive element 134, the volume of the reservoir 148 will tend to decrease as the first drive element 132 moves relative to the second drive element 134 when the former is acted upon by the drive spring 130. As the reservoir 148 collapses, damping fluid is forced through the vent 144 into the collection chamber 142. Thus, once the flexible latch arms 135 have been released, the force exerted by the drive spring 130 does work on the damping fluid, causing it to flow through the constriction formed by the vent 144, and also acts hydrostatically through the fluid and through friction between the first and second drive element 132, 134, to drive the second drive element 134. Losses associated with the flow of the damping fluid do not attenuate the force acting on the body of the syringe to a great extent. Thus, the return spring 126 remains compressed and the hypodermic needle 118 remains extended.
After a time, the second drive element 134 completes its travel within the syringe body 116 and can go no further. At this point, the contents of the syringe 114 are completely discharged and the force exerted by the drive spring 130 acts to retain the second drive element 134 in its terminal position and to continue to cause the damping fluid to flow through the vent 144, allowing the first drive element 132 to continue its movement.
Before the reservoir 148 of fluid is exhausted, the flexible latch arms 133 linking the drive sleeve 131 with the first drive element 132 reach another constriction 139, also provided by the component 162 that is attached to the syringe flange 120. The constriction 139 moves the flexible latch arms 133 inwards from the position shown to a position at which they no longer couple the drive sleeve 131 to the first drive element 132, aided by the bevelled surfaces on the constriction 139. Once this happens, the drive sleeve 131 acts no longer on the first drive element 132, allowing them to move relative to each other.
The latch arms 133 must be capable of supporting high shock load at the start of stroke, but must also be capable of releasing with relatively low unlatching forces. Tests have shown that this dual requirement is very difficult to achieve with flexible latch arms 133: if the latch arms are made stiff enough to carry the shock load, they may easily became too stiff to unlatch with acceptably small forces.
Once the drive sleeve 131 is acting no longer on the first drive element 132, of course, the syringe 114 is released, because the force developed by the drive spring 130 is no longer being transmitted to the syringe 114, and the only force acting on the syringe will be the return force from the return spring 126. Thus, the syringe 114 now returns to its retracted position and the injection cycle is complete.
All this takes place only once the cap 111 has been removed from the end of the housing 112 and the boot 123 from the syringe.